The project concerns the encoding of visual information at the retina and further processing of this information at higher levels in the visual pathway. The immediate objective is to understand as fully as possible the functional organization of the Limulus visual system in its natural state. The proposed experiments involve neuroanatomy, neurophysiology, biochemistry, and behavior. The results shall form the basis for extending and testing the present spatiotemporal model of neural integration in the retina and developing a quantitative description of neural interactions in the brain. Strong emphasis is placed on the mechanisms that underlie the efferent control of retinal sensitivity in Limulus. We recently discovered that efferent optic nerve fibers mediate circadian changes in photoreceptor structure and sensitivity. This discovery opens up a new area of research in the functional organization of the visual system. It provides a model system for studying the cellular mechanisms that control visual sensitivity. It also provides a system for investigating the neurosecretory mechanisms that mediate the effects of the endogenous circadian clock. The Limulus visual system will be investigated under as normal physiological conditions as possible. Experiments will be performed on the eye and brain in situ and in organ-culture media. Our ability to record intracellularly from single photoreceptors in situ for periods of up to 12 hours and extracellularly from single optic nerve fibers in situ for up to 6 days enables us to perform experiments that are not yet possible in any other visual system. Our long-range goal is to use the results of our research as a guide for understanding more complex visual systems.